Case Hardening Method for High Performance Long Life Martensitic Stainless Steel Bearings

ABSTRACT

A method for cost effectively case hardening a component formed from a martensitic stainless steel material with a desired metallurgical condition for high temperature, high rolling contact fatigue, corrosion and spall initiation and propagation resistance bearing performance. The method describes a method to significantly reduce the carburization or carbo-nitriding process times for appreciable reduction in manufacturing cost. The Method includes the steps of: forming the component from a martensitic stainless steel material having an ASTM grain size of 9 or finer; and subjecting the component to one of a carburization and a carbo-nitriding treatment with significantly lower case hardening times for manufacturing cost-effectiveness.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of provisional application Ser. No.61/908,275, filed Nov. 25, 2013.

BACKGROUND

The present disclosure relates to a case hardening method for highperformance long life martensitic stainless steel bearings.

Modern aerospace bearings are required to sustain and transfer highstresses from radial and/or axial loads under rapid rotational motion,over a range of temperatures from sub-zero to those that challenge thecapability of the lubricant and steel materials. As such, the steelmaterial has to have high load bearing capability (strength andhardness) and other characteristics that include, but are not limitedto, temperature capability, fracture toughness, wear resistance, andoil-out capability. One key material characteristic that is embodied inthose fundamental requirements is the rolling contact fatigue (RCF)endurance limit of the steel. This characteristic is used in the designand lifting of mechanical systems components (bearings and gears).

The RCF endurance limit of the bearing steels is defined as thematerials capability limit when a surface spall initiation event occurs.That event is caused by a combination of independent bearing operatingvariables, stress, temperature and rolling contact fatigue cycles/time.

Martensitic steels that are suited for over 95% of high performancebearings and gears due to their unique balance of physical andmechanical properties: moderately high elastic modulus, thermalconductivity, ultra-high hardness and compressive strength, limited tomoderate fracture toughness, temperature capability to approximately 600F, and rolling contact fatigue resistance.

Current hardening processes for martensitic stainless steels take over100 hours of carburization time in high cost furnaces like vacuumcarburization or plasma carburization. Such process times are more thantwo times required for conventional case hardening non-stainlessmartensitic steels, and at proportionally higher cost. The costnegatively impacts the cost of component design to cost metrics.

There is a need for a method which significantly reduces case hardeningtime for high performance long life martensitic stainless steelbearings.

SUMMARY

In accordance with the present disclosure, there is provided a methodfor case hardening high performance long life martensitic stainlesssteel components such as bearings.

In accordance with the present disclosure, there is provided a methodfor case hardening a component formed from a martensitic stainless steelmaterial, which method broadly comprises the steps of: forming acomponent from a martensitic stainless steel material having a grainsize of ASTM (American Soc. For the Testing of Materials) 9 or finer;and subjecting the component to one of a carburization and acarbo-nitriding treatment.

In another and alternative embodiment, the component forming step maycomprise providing a martensitic stainless steel material having a grainsize of at least 5 and thermo-mechanically processing the martensiticstainless steel material to have a grain size of 9 or finer.

In another and alternative step, the thermo-mechanically processing stepmay comprise subjecting the martensitic stainless steel material to afirst reduction of at least 50%, a second reduction of at least 60% anda third reduction of at least 70%.

In another and alternative step, each of the first, second and thirdreductions may be performed by forging or ring rolling at an elevatedtemperature using a strain rate of 5×10⁻¹ inch/inch/second or higher.

In another and alternative embodiment, the first reduction may beperformed at a grain coarsening temperature of the martensitic stainlesssteel material, the second reduction may be performed at a temperature<1850 F, and the third reduction may be performed at a temperature abovethe recrystallization temperature of the martensitic stainless steelmaterial and below the temperature at which the second reduction isperformed.

In another and alternative embodiment, the subjecting step may comprisesubjecting the component to a carburization process.

In another and alternative embodiment, the subjecting step may comprisesubjecting the component to a carbo-nitriding process.

Further in accordance with the present disclosure, a method for forminga martensitic stainless steel material may broadly comprise the stepsof: providing a martensitic stainless steel material; andthermo-mechanically processing the martensitic stainless steel materialto have a grain size of 9 or finer.

In another and alternative embodiment, the thermo-mechanicallyprocessing step may comprise subjecting the martensitic stainless steelto a first reduction of at least 50%, a second reduction of at least 60%and a third reduction of at least 70%.

In another and alternative embodiment, each of the first, second, andthird reductions may be performed by forging or ring rolling at anelevated temperature using a strain rate in the range of 5.0×10⁻¹inch/inch/second or higher.

In another and alternative embodiment, the first reduction may beperformed at a higher temperature of 1900 to 1850 F of the martensiticstainless steel material, the second reduction may be performed i^(n) atemperature range of 1850 to 1775 F, and the third reduction may beperformed at a temperature above the recrystallization temperature ofthe martensitic stainless steel material and below the temperature atwhich the second reduction is performed.

Further, in accordance with the present disclosure, there is provided acomponent which broadly comprises: a core formed from a martensiticstainless steel material having a grain size of 9 or coarser in thefinal heat treat condition; and a case hardening structure surroundingthe core.

Other details of the method for case hardening high performance longlife martensitic stainless steel components are set forth in thefollowing detailed description and the accompanying drawings whereinlike reference numerals depict like elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a Fe—Cr—C—X isopleth phase diagram showing the preferredtemperature for carburization of a stainless steel such as PYROWEAR 675;

FIG. 2 is a schematic illustration showing the relative rates of carbondiffusion in a martensitic stainless steel structure;

FIG. 3 is a plot of average grain diameter and number of grains per unitvolume versus grain size;

FIG. 4 is a plot of grain boundary area as a function of grain size;

FIG. 5 is a photomicrograph of coarse grained ASTM 3 PYROWEAR 675 withunacceptable microstructure: coarse carbides that necklace the grain andtwin boundaries;

FIG. 6 is a photomicrograph of coarse grained ASTM 3 PYROWEAR 675 withunacceptable microstructure: coarse carbides that necklace the grain andtwin boundaries;

FIG. 7 is a schematic illustration of the thermo-mechanical processingfor producing uniform fine grain rings and rolling elements; and

FIG. 8 is a plot showing PYROWEAR 675 grain size as a function ofexposure temperature.

DETAILED DESCRIPTION

The basis of the present disclosure is a method which significantlyreduces the case hardening time to produce an adequate case depth foraerospace main shaft bearings with acceptable metallurgical condition.The reduced case hardening times directly and positively impact themanufacturing cost of the bearing.

The technical approach to the cost effective case hardening method formartensitic stainless steels is based on (a) thermodynamics and (b) thekinetics of the martensitic stainless steel.

With regard to the thermo-dynamics for carburization or carbo-nitridingcase hardening of a class III super 12% chromium martensitic stainlesssteel with acceptable metallurgical condition, a martensitic stainlesssteel may be chosen whose compositional make-up favors the formation ofmoderately stable M23C6 carbides over the thermo-dynamically more stableMC and M7C3 carbides.

A suitable martensitic stainless steel may have a composition consistingof from 8.0 to 18 wt % chromium, up to 16 wt % cobalt, up to 5.0 wt %vanadium, up to 8.0 wt % molybdenum, up to 8.0 wt % nickel, up to 4.0 wt% manganese, up to 2.0 wt % silicon, up to 6.0 wt % tungsten, up to 2.0wt % titanium, up to 4.0 wt % niobium, and the balance iron AND meet theprovisions of [0031].

There are a number of steels that can be used in the present invention,e.g. PYROWEAR 675 made by Carpenter Technologies having a composition inwt %: Fe-13 Cr-5.4Co-1.8Mo-2.6Ni-0.6Mn-0.6V-0.4Si-0.07C; CSS-42L made byLatrobe Steel having a composition in wt %:Fe-14Cr-12.4Co.4.7Mo-2.05Ni-0.6V-0.05C; and AFC-77 made by CrucibleResearch.

Other alloys which may be used have a composition in wt % as follows:

-   -   Fe-13.75Cr-5Co-3Mo-3Ni-0.08V-0.75Mn-0.4Si-0.15C;    -   Fe-14Cr-5Co-4Mo-3.5Ni-0.08V-0.22Mn-0.3Si0.15C;    -   Fe-13.5Cr-3.75Co-3.5Mo-3Ni-0.08V-0.25Mn-0.3Si-0.15C;    -   Fe-13.5Cr-3.75Co-3.5Mo-3Ni-1Ti-1Mn-0.3Si-0.15C;    -   Fe-15.25Cr-5Co-3.5Mo-4Ni-0.25V-0.2Mn-0.25Si-0.15C; and    -   Fe-14Cr-2.75Co-3.25Mo-3.5Ni-0.3V-0.3Mn-0.3Si-0.15C.

The selected steels may then be case hardened by carburization orcarbo-nitriding in a temperature/composition field including carbon orcarbon+nitrogen that ranges from the base composition to the aimelevated carbon or carbon+nitrogen in the case hardened region, wherethere is a preference for the formation of moderate thermodynamicstability carbides like M23C7, M6C, or M2C, where M represents a metalatom and C represents a carbon atom, or metal-carbo-nitride. This isshown in FIG. 1 which is a Fe—Cr—C—X Isopleth Phase Diagram showing thepreferred temperature for carburization of a steel, such as Pyrowear675, where a temperature of 1700 F (927 C)(line A) is a yes and atemperature of 1975 F (1079 C)(line B) is a no.

A key to successful case hardening of martensitic stainless steels to anacceptable metallurgical condition for high performance is the kineticsof carbon or carbon+nitrogen addition to the surface to be hardened to aprescribed depth. Carbon and nitrogen are elements that significantlyharden and strengthen the steel martensite phase for increased loadbearing capability. To meet this objective, the method must allow auniform diffusion of carbon or carbon+nitrogen into the steel to theprescribed depth. That is, no localized build up of those elementsanywhere in the steel microstructure during or after the case hardeningtreatment. To accomplish this, one has to recognize the differentdiffusion paths for the carbon and nitrogen in the steel structure andthe relative rates of each path. In order of diffusion ease or speed,fastest are (1) grain boundaries, then (2) twin boundaries and theslowest diffusion is by (3) bulk diffusion through the crystal structureof the grain. This is schematically shown in FIG. 2.

As shown in FIG. 2, carbon atom diffusion in a polycrystallinestructure, from a highest to lowest rate is open structured grainboundaries 10 (fast), twin boundaries 12 (moderate) and bulk diffusionin grains 14 (very slow). Refractory alloying additions to stainlesssteel retard diffusion further.

Based on the fundamental microstructure related diffusion ratesdescribed above, the technical approach embodied in this disclosure forsignificantly reduced case hardening process times is to increasediffusion rates as provided by a finer grain structure. That is,increased ground boundary area per unit volume for easy carbon orcarbon+nitrogen to diffuse uninhibited. For any retardation of carbondiffusion on the grain boundaries will result in significant localizedbuild up of those elements and the formation of unacceptable chunkycarbides like MC and/or M7C3 or carbo-nitrides. The grain size to (1)average grain diameter and number of grains per unit volume and (2)grain boundary area per unit volume is shown in FIGS. 3 and 4 and table1 respectively.

FIG. 3 is a plot of average grain diameter and number of grains per unitvolume versus grain size. FIG. 4 is a plot of grain boundary area as afunction of grain size.

Table I is a tabulation of grain characteristics as a function of grainsize.

Average Grain Surface ASTM Grain Number of Average Grain Diameter, areaSize Number Grains/mm3 Diameter, mm microns per mm3 −2 2 0.75 750 3.76−1 5.6 0.5 500 6.29 0 16 0.35 350 10.64 1 45 0.25 250 17.84 2 128 0.18180 30.08 3 360 0.125 125 50.45 4 1020 0.091 91 84.92 5 2900 0.062 62143.19 6 8200 0.044 44 240.78 7 23000 0.032 32 403.26 8 65000 0.022 22677.91 9 185000 0.016 16 1143.68 10 520000 0.011 11 1917.43 11 15000000.008 8 3256.6 12 4200000 0.006 6 5449.33

Example I

This example is directed to stainless steel with an ASTM grain size of 3to 4. Many manufactures have tried but few have succeeded in processingsuch a material. This is because they typically hot process themartensitic stainless steel at about 1900 F to 2000 F. The result is arelatively coarse grained condition of grain size ASTM #3 to 4, thatwhen carburized case hardened produces an unacceptable metallurgicalmicrostructure. That unacceptable metallurgical microstructure is shownin FIG. 5. Referring to FIG. 4 and Table I, the grain size 4 has 1020grains per cubic centimeter. That amount of grain boundary area isinsufficient for rapid carbon or carbon+nitrogen and they become locallysaturated, resulting in the unacceptable coarse carbide formationobserved in the grain and twin boundaries.

Example II

This example is directed to stainless steel with an ASTM grain size of 7to 8. When this grain size is attained in the processed stainless steel,it can be successfully carburized case hardened to an acceptablemetallurgical microstructure as shown in FIG. 6, and described as auniform dispersion of moderate to fine carbides in a fine grainstructure. That is, no unacceptable coarse carbide networking anywherein the microstructure.

Grain statistics for the grain size 7 are as follows. Grain size 7 has23,000 grains per cubic centimeter compared to 1020 for grain size 4,and a grain boundary area of 403 square centimeter per cubic centimeteras compared to 85 for the grain size 4 condition; that is a 4.7×increased grain boundary area. That amount of grain boundary area issufficient for rapid carbon or carbon+nitrogen without any localsaturation, resulting in the acceptable microstructure shown in FIG. 6.

Despite that success (grain size 7 to 8), the carburization time to meetthe aerospace case depth requirement necessitates process times inexcess of 100 hours in specialized expensive systems, resulting incostly case hardening as compared to conventional non-stainlessmartensitic steels.

In accordance with the present disclosure, cost effective case hardening(significantly reduced carburization times) is accomplished byincreasing the grain boundary area per unit volume of steel to providean even greater easy diffusion path for the carbon or carbon+nitrogen todiffuse into the stainless steel without any localized build up of thoseelement(s). The method embodied herein is to process the martensiticstainless steel to a grain size 9 or finer. This can be accomplishedthrough a sequential thermo-mechanical process schedule, whereprocessing starts at 1900 F (1038 C) maximum, and the steel is processedper schematic shown in FIG. 7 with a finishing temperature of 1725 F(940 C) or lower. In doing so, the grain size of ASTM 9 can be achieved.

As shown in FIG. 7, one may begin with a starting stock of martensiticstainless steel having a grain size of 5. Thereafter, one may firstforge the stainless steel by performing a reduction of approximately 50%at the grain coarsening temperature of the stainless steel at a strainrate of at least 5.0×10⁻¹ inch/inch/second. One may take a 5 to 20%reduction per pass according to the capability of the processingequipment). The first reduction may be taken at a temperature of from1850 to 1900 degrees F. In a second step, the martensitic stainlesssteel is forged to a reduction of greater than or equal to 60% at atemperature in the range of from 1775 to 1850 degrees F. at the samestrain rate as set out above. In a final step, one then forges thestainless steel to a reduction of greater than or equal to 70% at atemperature slightly above the recrystallization (Rx) temperature at thestrain rate set forth hereinbefore. The temperature may be >25 F abovethe Rx temperature to allow for process variations of the stainlesssteel. This yields a martensitic stainless steel which has a grain sizein the range of 9 or finer. This thermo-mechanical process enables oneto use higher strain rates without a significant compromise ofductility. This thermo-mechanical process enables one to produce uniformfine grain rings and rolling elements.

FIG. 8 illustrates PYROWEAR 675 grain size as a function of heattreatment exposure temperature for a 4 hour soak.

Grain size 9 has 185,000 grains per cubic centimeter compared to 23,000for grain size 7, i.e. an 8× increase. Grain boundaries of 1144 squarecentimeter per cubic centimeter are present in a grain size 9 conditionas compared to 403 for the grain size 7 condition. That is a 2.8×increase in grain boundary area. That increased grain boundary areaallows even more rapid carbon or carbon+nitrogen without any localsaturation, for acceptable microstructure as shown in FIG. 6. Thecarburization times can be reduced by at least 50%.

After the martensitic stainless steel having a grain size in the rangeof 9 or finer is produced, it may be shaped into a product such as abearing or a gear. The product may be machined into a rough form.

After being formed into a desired product form, the stainless steelmaterial may be subjected to a carbo-nitriding process. Processescapable of carbo-nitriding the aforementioned martensitic stainlesssteel material to a desired condition include, but are not limited to:

A carbo-nitriding process at a pressure of 1 atmosphere or lower and atemperature which varies with steel composition and is typically in therange of 1650 to 2000° F. for a time which varies according to desiredcase depth, from 40 to 200 hrs., typically. The atmosphere compositionhas carbon and nitrogen “potentials” as indicated below. Typically, thecarbide and nitrogen (C+N) levels are less than the aim level due to a“gettering” (kinetic effort of getting more C and N from the source)effect by the steel during carbo-nitriding process;

Vacuum carbo-nitriding process at a pressure and a temperature whichvaries with steel composition, typically in the range of 1650 to 2000°F. for a time which varies according to the desired case depth, from 40to 200 hrs. typically. The atmosphere composition has carbon andnitrogen “potentials” as indicated below. Typically, the C+N levels areless than the aim level due to a “gettering” effect by the steel duringcarbo-nitriding process; and

Plasma carbo-nitriding process at a pressure and a temperatures whichvaries with steel composition, typically in the range of 1650 to 2000°F. for a time which varies according to desired case depth, from 40 to200 hrs., typically. The atmosphere composition has carbon and nitrogen“potentials” as indicated below. Typically, the C+N levels are less thanthe aim level due to a “gettering” effect by the steel duringcarbo-nitriding process.

The above processes may be conducted to produce prescribed levels ofcarbon (0.2 to 0.55 wt %) and nitrogen (0.2 to 1.2 wt %) in the hardenedcase for attaining goal hardness and corrosion resistance as follows:

Total carbon+nitrogen in the range of from 0.5 to 1.7 wt % for hardnessdesired case hardness;

Carbon+nitrogen levels in treated surface case limited to ensurechromium content of carbo-nitrided case >6 wt %, for good corrosionresistance. An example of this requirement is indicated below. For 0.35wt % carbon+0.4 wt % nitrogen, the chromium content of the matrix is >6wt %.

A carburization process, such as vacuum or plasma-assisted carburizationmay be used to introduce the prescribed amount of carbon into thesurface of the steel product. Vacuum or plasma assisted carburizationallows the diffusion rate of the carbon to be more readily controllableto achieve the desired uniform dispersion of carbon into the surface ofthe steel product. Gas carburization may also be used.

After carburizing the product may be selectively machined. This isfollowed by austenitzing, quenching, subzero cooling and tempering toachieve the desired hardness and microstructure. The steel product isfinish machine to conform to design configuration.

The case hardened structure formed in accordance with the presentdisclosure will be the same as described in earlier sections, i.e. auniform fine dispersion of carbides or carbo-nitirides in the hardenedcase. However, it should be noted that the as-case carburized oras-carbo-nitrided condition may have slightly coarser carbide orcarbo-nitride precipitates than the later harden+tempered condition ofthe final product. The subsequent Harden Heat Treatment relies of thesolubility of M23C6 carbides or carbo-nitrides at the hardentemperature, followed by controlled precipitation during the tempertreatment to a finer dispersion.

There has been provided a case hardening method for high performancelong life martensitic stainless steel bearings. While the case hardeningmethod for high performance long life martensitic stainless steelbearings has been described in the context of specific embodimentsthereof, unforeseen alternatives, modifications, and variations maybecome apparent to those skilled in the art having read the foregoingdescription. Accordingly, it is intended to embrace those alternatives,modifications, and variations as fall within the broad scope of theappended claims.

What is claimed is:
 1. A method for case hardening a component formed from a martensitic stainless steel material, which method comprises the steps of: forming said component from a martensitic stainless steel material having an ASTM grain size of 9 or finer; and subjecting said component to one of a carburization and a carbo-nitriding treatment.
 2. The method of claim 1, wherein said component forming step comprising providing a martensitic stainless steel material having a grain size of at least 5 and thermo-mechanically processing said martensitic stainless steel material to have a grain size of 9 or finer.
 3. The method of claim 2, wherein said thermo-mechanically processing step comprises subjecting said martensitic stainless steel to a first reduction of at least 50%, a second reduction of at least 60% and a third reduction of at least 70%.
 4. The method of claim 3, wherein each of said first, second, and third reductions is performed by forging or ring rolling at an elevated temperature using a strain rate in the range of 5.0×10⁻¹ inch/inch/second or higher.
 5. The method of claim 3, wherein said first reduction is performed at a higher temperature of 1900 to 1850 F of said martensitic stainless steel material, said second reduction is performed at a temperature range of 1850 to 1775 F, and said third reduction is performed at a temperature above the recrystallization temperature of the martensitic stainless steel material and below the temperature at which the second reduction is performed.
 6. The method of claim 1, wherein said subjecting step comprises subjecting said component to a carburization process.
 7. The method of claim 1, wherein said subjecting step comprises subjecting said component to a carbo-nitriding process.
 8. A method for forming a martensitic stainless steel material comprising the steps of: providing a martensitic stainless steel material; and thermo-mechanically processing said martensitic stainless steel material to have an ASTM grain size of 9 or finer.
 9. The method of claim 8, wherein said thermo-mechanically processing step comprises subjecting said martensitic stainless steel to a first reduction of at least 50%, a second reduction of at least 60% and a third reduction of at least 70%.
 10. The method of claim 9, wherein each of said first, second, and third reductions is performed by forging or ring rolling at an elevated temperature using a strain rate in the range of 5.0×10⁻¹ inch/inch/second or higher.
 11. The method of claim 9, wherein said first reduction is performed at a higher temperature of 1900 to 1850 F of said martensitic stainless steel material, said second reduction is performed at a temperature range of 1850 to 1775 F, and said third reduction is performed at a temperature above the recrystallization temperature of the martensitic stainless steel material and below the temperature at which the second reduction is performed.
 12. A component comprising: a core formed from a martensitic stainless steel material having a grain size of 9 or finer in the as processed condition and a coarser grain size in the final harden and temper heat treat condition; and a case hardening structure surrounding said core. 